border opacity
div { border: 1px solid rgb(127, 0, 0); border: 1px solid rgba(255, 0, 0, .5); -webkit-background-clip: padding-box; /* for Safari */ background-clip: padding-box; /* for IE9+, Firefox 4+, Opera, Chrome */ }
出处:http://stackoverflow.com/questions/4062001/css3-border-opacity